Batteries used in stand alone power supply systems are commonly lead-acid batteries. However, lead-acid batteries have limitations in terms of performance and environmental safety. For example, typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal.
Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer a potential to overcome the above mentioned limitations of lead-acid batteries. In particular, the useful lifetime of flowing electrolyte batteries is not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
A flowing electrolyte battery, like a lead acid battery, comprises a stack of cells that produce a total voltage higher than that of the individual cells. But unlike a lead acid battery, cells in a flowing electrolyte battery are hydraulically connected through an electrolyte circulation path.
Referring to FIG. 1, a flow diagram illustrates a basic zinc-bromine flowing electrolyte battery 100, as known according to the prior art. The zinc-bromine battery 100 includes a negative electrolyte circulation path 105 and an independent positive electrolyte circulation path 110. The negative electrolyte circulation path 105 contains zinc ions as an active chemical, and the positive electrolyte circulation path 110 contains bromine ions as an active chemical. The zinc-bromine battery 100 also comprises a negative electrolyte pump 115, a positive electrolyte pump 120, a negative zinc electrolyte (anolyte) tank 125, and a positive bromine electrolyte (catholyte) tank 130.
To obtain high voltage, the zinc-bromine battery 100 further comprises a stack of cells connected in a bipolar arrangement. For example, a cell 135 comprises half cells 140, 145 including a bipolar electrode plate 155 and a micro porous separator plate 165. The zinc-bromine battery 100 then has a positive polarity end at a collector electrode plate 160, and a negative polarity end at another collector electrode plate 150.
A chemical reaction in a positive half cell, such as the half cell 145, during charging can be described according to the following equation:2Br−→Br2+2e−  Eq. 1Bromine is thus formed in half cells in hydraulic communication with the positive electrolyte circulation path 110 and is then stored in the positive bromine electrolyte tank 130. A chemical reaction in a negative half cell, such as the half cell 140, during charging can be described according to the following equation:Zn2++2e−→Zn  Eq. 2A metallic zinc layer 170 is thus formed on the collector electrode plate 150 in contact with the negative electrolyte circulation path 105. Chemical reactions in the half cells 140, 145 during discharging are then the reverse of Eq. 1 and Eq. 2.
Some prior art electrode plates are manufactured from polymers mixed with conductive fillers, such as graphite or carbon black. The polymer and the conductive filler are mixed at a relatively high temperature and pressed or extruded into a desired shape, typically a sheet. Activated carbon particles or activated carbon cloth or felt is then heat bonded onto the sheet to provide a large conductive surface area. Activated carbon granules may also be attached with conductive adhesive and heat curing. For zinc-bromide or zinc-chloride batteries the cathode is prepared as described above. On the anode where zinc metal is plated during charging, the conductive plastic polymer also serves as the electrode surface for the zinc plating process.
However, a problem with electrode plates manufactured from polymers mixed with conductive fillers is that the surface conductivity of the conductive mix can significantly degrade over time. This can, for example, be due to thermal factors, or due to the acidic or oxidative nature of an electrolyte and the associated electrochemical reactions. To extend the life of these electrodes, increased loadings of conductive filler must be added to what is otherwise needed on new electrodes. This makes the process of manufacturing the sheet more difficult and results in a trade-off of mechanical toughness against the increased carbon loading. Also, this leads to a second problem with these types of electrode plates which concerns a high cost associated with conductive fillers and the plate manufacturing process.
Another example of a prior art bipolar electrode for a flowing electrolyte battery is described in international patent application no. PCT/AU00/00241 to Hagg et al (International Publication no. WO 00/57507). Hagg et al describe an electrode manufactured using an alternative process involving a non-conductive polymer. Pieces of graphite felt are pressed onto both sides of a non-conductive polymer sheet such that the pieces of felt make contact with each other in the middle of the sheet. However, this method has the disadvantage that oxidation can occur at contact points of the felt, which contact points are where graphite fibres of the graphite felt extending from opposite sides of the polymer sheet contact each other in the middle of the sheet. For zinc-bromide battery applications, the zinc plating surface must be smooth and of uniform conductivity to facilitate acceptable zinc plating quality. In this example, the zinc side felt would have to be pressed fully into the non-conductive polymer. This leads to poor zinc plating quality because the conductivity of the surface is not uniform enough due to the relatively large dimensions of the fibres.
There is therefore a need to overcome or alleviate many of the above discussed problems associated with flowing electrolyte batteries of the prior art.